Optical projection exposure tools are now widely used in optical lithography in the production of integrated circuits. The exposure tools generally comprise an electromagnetic light source, a mask and a lens. The tool projects the image of the mask onto the surface of a photoresist film disposed on a wafer. Because of the continuing demands for higher circuit density in integrated circuits, there is a continuing desire to improve the resolution of optical lithography. One method of improving the resolution involves the use of phase-shifting masks. The phase-shifting mask generally comprises a plurality of individual transmissive phase-shifting layers disposed in a pattern on a transmissive substrate. Optionally, the phase-shifting mask can also be provided with an opaque chrome pattern. The phase-shifting layer causes the transmitted light to be out of phase (phase shifted) with respect to the light which is only transmitted through the substrate, i.e. the reference light, resulting in coherent interaction of the phase-shifted and reference transmitted light. Depending on the pattern of the phase shifting layers, the coherent interaction of the transmitted phase-shifted and reference light can be (i) completely destructive interference which prevents light from reaching the surface of the photoresist film or (ii) a combination of constructive and destructive interference which improves the resolution of the light which reaches the surface of the photoresist film. Phase-shifting masks are generally fabricated using standard photolithography techniques. For example, Levenson et al., in his article "Improving Resolution in Photolithography with Phase-Shifting Mask" IEEE Transaction on Electron Devices Vol. 29, No. 12 (12/82), discloses fabrication of a phase-shifting mask using photolithography with polymethacrylate as the mask material. However, occasionally during the fabrication of the phase-shifting mask, defects, such as voids, occur in the mask. These defects disrupt the coherent interaction of the transmitted light. For regions of the mask designed (patterned) to prevent light from reaching the photoresist (nonprinting regions of the mask) the defects can result in leakage of light onto the surface of the photoresist film. Therefore, there is a need in the art for a method for repairing defects in the nonprinting regions of phase-shifting mask.
It is therefore an object of the present invention to provide a method for repairing defects in the nonprinting regions of phase-shifting masks.
Other objects and advantages will become apparent from the following disclosure.